In wireless communication systems in recent years, studies on an adaptive communication scheme are underway whereby a modulation scheme and a coding scheme are adaptively switched according to communication quality of radio communication links. For example, Non-Patent Document 1 discloses a communication scheme using adaptive modulation switching, which is an example of the adaptive communication scheme. As information for deciding switching of a modulation scheme or a coding scheme, communication quality of communication links is generally measured and used.
As an index indicating communication quality, there is, for example, a method of estimating and using a bit error rate (BER) in a received bit data sequence. Patent Document 1 discloses an example of such a method whereby a transmission data sequence is subjected to error correction coding, error correction decoding upon reception and then re-encoding, and this re-encoded data sequence is compared with the received signal sequence, and the number of different parts is calculated by counting.
FIG. 11 shows the configuration of a conventional bit error rate estimating apparatus and is an example of an apparatus that measures communication quality.
In FIG. 11, bit error rate calculating apparatus 10 employs a configuration having demodulation section 11, decoding section 12, memory section 13, re-encoding section 14 and bit error rate calculating section 15. In a communication system using this conventional bit error rate calculating apparatus 10, it is assumed that transmission data is subjected to predetermined error correction coding in advance, modulated and transmitted and inputted to bit error rate calculating apparatus 10 as received signal 1. Furthermore, it is assumed that convolutional coding is used as an example of predetermined error correction coding.
Received signal 1 is demodulated according to a predetermined modulation scheme at demodulation section 11, and the demodulated result is outputted. Here, it is assumed that soft decision value 2a and hard decision value 2b are outputted per bit as the demodulated result. Soft decision value 2a for each bit is supplied to decoding section 12, error-corrected through maximum likelihood decoding such as Viterbi decoding, and decoded bit data sequence 3 is outputted. By contrast, hard decision value 2b is supplied to memory section 13 and stored temporarily. Bit data sequence 3 outputted from decoding section 12 is supplied to re-encoding section 14, subjected to convolutional coding similar to that on the transmitting side, and re-encoded data sequence 5 is outputted. Bit error rate calculating section 15 makes a comparison in bit units between hard decision data sequence 4, which is temporarily stored in and read out from memory section 13, and re-encoded data sequence 5 to judge whether or not bits in the two sequences are equal, counts unequal bits as bits containing bit errors and finally outputs calculated result 6 of the bit error rate.
Non-Patent Document 1: “Mobile Communication” written and edited by Shuichi Sasaoka (Ohmsha, Ltd. P. 103 to 126)
Patent Document 1: Japanese Patent Application Laid-Open No. SHO61-135234